The invention relates to a ballast for operating at least one low-pressure discharge lamp.
U.S. Pat. No. 4,392,085 discloses a ballast for operating two parallel-connected low-pressure discharge lamps on a half-bridge inverter. The ballast has a transformer with two similar windings that are each connected to one of the two lamp branch circuits. This transformer operates as a differential current transformer. If one of the two lamps ignites earlier than the other, the ignition of the other lamp is supported with the aid of the transformer.
The problem frequently arises in the case of ballasts for operating a plurality of low-pressure discharge lamps that the lamps shine with a different brightness in the dimming mode. If the lamps are arranged in different, parallel-connected branch circuits, this problem can be caused by different currents in the branch circuits. On the other hand, however, this problem also occurs with lamps thatxe2x80x94as in the case of a series connection of two lampsxe2x80x94are arranged in the same branch circuit. In particular, when the ballast has an inverter that is provided with a downstream load circuit designed as a resonant circuit, in which the lamps are connected, those lamps that are directly connected to the resonant circuit components frequency shine more brightly than the downstream lamps. In addition, it may be observed that when only one low-pressure discharge lamp is operated in each branch circuit the two ends of the lamp shine with a different brightness. These brightness differences become more obvious the stronger the lamps are dimmed. For the abovementioned reasons, in the case of conventional operating devices or ballasts, in particular for the series connection of a plurality of low-pressure discharge lamps, the lower dimming stage or brightness stage is limited to approximately 10% of a maximum brightness. The aforesaid problems are caused by parasitic capacitances between the lamp or the lamp leads and the luminaire and also by parasitic capacitances inside the heating circuit.
It is the object of the invention to provide a ballast in the case of which the above-named problems no longer occur.
This object is achieved by the means of a ballast having the features of patent claims 1, 2, 4 or 7. Particularly advantageous embodiments of the invention are disclosed in the dependent patent claims.
For low-pressure discharge lamps that are arranged in three parallel-connected branch circuits, the object is achieved by means of a ballast having the features of patent claim 1 or, alternatively, having the features of patent claim 2. For low-pressure discharge lamps that are arranged in two parallel-connected branch circuits, the object of the invention is achieved by means of a ballast having the features of patent claim 4. For low-pressure discharge lamps that are arranged in the same branch circuit, that is to say for a series connection of a plurality of low-pressure discharge lamps, or for the operation of a single low-pressure discharge lamp, the object of the invention is achieved by means of a ballast having the features of patent claim 7.
In accordance with a first variant of the invention, the ballast has three parallel-connected branch circuits to which in each case electric terminals for at least one low-pressure discharge lamp are connected. Moreover, this ballast has the following further features:
a transformer for balancing the lamp currents, the transformer having a first and a second winding, and the first winding having twice as many turns as the second winding,
a second transformer for balancing the lamp currents, the second transformer having a first and second winding, and the first winding having twice as many turns as the second winding,
the windings of the transformers being arranged in such a way that when the low-pressure discharge lamps are connected the lamp currents flow firstly through the windings of one of the two transformers, then through the low-pressure discharge lamps and only then through the windings of the other transformer,
the first winding of the first transformer being arranged in such a way that when the low-pressure discharge lamps are connected it is flowed through by the lamp current of the low-pressure discharge lamp or low-pressure discharge lamps connected to the first branch circuit,
the second winding of the first transformer being arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp currents of the low-pressure discharge lamps connected to the second and third branch circuit, the two windings of the first transformer being wound in opposite senses,
the first winding of the second transformer being arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp current of the low-pressure discharge lamp or low-pressure discharge lamps connected to the third branch circuit, and
the second winding of the second transformer being arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp currents of the low-pressure discharge lamps connected to the first and second branch circuit, the two windings of the second transformer being wound in opposite senses.
This variant of the invention is particularly well suited for operating three parallel-connected low-pressure discharge lamps. It is distinguished by a particularly simple wiring of the luminaire. The two transformers act like a differential current transformer owing to the arrangement of their first and second windings in opposite senses. Owing to the special turn ratio of 2:1 between the first and second windings for both transformers, it is ensured that all three branch circuits are of balanced design, since the second windings of the two transformers are flowed through by the partial currents of two branch circuits. However, owing to the special arrangement of the windings of the two transformers, it is ensured that the currents in the three branch circuits are balanced, on the one hand, and also that the losses owing to parasitic capacitances in each individual branch circuit or in each individual lamp are compensated, on the other hand. Consequently, all three lamps shine with the same brightness even in the event of strong dimming.
In accordance with a second variant of the invention, the ballast has three parallel-connected branch circuits to which electric terminals for at least one low-pressure discharge lamp are connected in each case. Moreover, this ballast has a transformer with three identically constructed and isotropically arranged windings that are arranged in each case in one of the branch circuits such that they are flowed through by the lamp current in the same sense. In this context, isotropic means that the transformer is equipped with a three-limb core and three identical windings, one winding being arranged on each limb. These windings are arranged in this case in the branch circuits in such a way that they are flowed through by the lamp current in the same sense. Owing to this arrangement of the windings, and to the identical design of the limbs, magnetic flux induced through each winding is distributed uniformly over the other limbs. A change in the current in one of the three branch circuits therefore automatically effects a corresponding change in the current in the two other branch circuits. If more than one low-pressure discharge lamp is arranged in one of the three branch circuits, for example a series connection composed of two low-pressure discharge lamps, the ballast is advantageously equipped with a second transformer for balancing the lamp current, the two windings of this second transformer having the same number of turns per unit length and being connected upstream and downstream of the parallel connection of the three branch circuits such that the windings are flowed through by the total current of the branch circuits in the opposite senses. This measure ensures that all the lamps that are arranged in the same branch circuit shine with the same brightness.
In accordance with a third variant of the invention, the ballast has the following features:
a transformer for balancing the lamp currents,
two parallel-connected branch circuits to which electric terminals for at least one low-pressure discharge lamp are connected in each case,
the transformer having a first winding that is connected to the first branch circuit, and having a second winding with the same number of turns that is connected to the second branch circuit with the opposite winding sense,
a second transformer for balancing the lamp currents,
the second transformer having a first and a second winding with the same number of turns per unit length, the first winding being arranged upstream of the parallel connection of the branch circuits, and the second winding being arranged downstream of the parallel connection of the branch circuits, and
the first and the second windings of the second transformer being arranged in such a way that, with the low-pressure discharge lamps connected, they are flowed through in the opposite sense by the total current of the parallel connection of the branch circuits.
Both transformers act as differential current transformers. The first transformer ensures balancing of the currents in the two parallel-connected branch circuits, while the second transformer causes balancing of the total current upstream and downstream of the parallel connection of the branch circuits. The first transformer ensures that the lamps in the two branch circuits shine with the same brightness, while the second transformer ensures that the lamps arranged in the same branch circuitxe2x80x94that is to say the series-connected lamps of each branch circuitxe2x80x94shine with the same brightness. The second transformer compensates the losses that are produced by parasitic capacitances. In its attempt to cause equal currents to flow in its windings, it balances the leakage currents caused by parasitic capacitances. In this process, it lowers the potential of the lamp terminal assigned to the capacitor C1, while it correspondingly raises the potential of the lamp terminal assigned to the capacitor C2. This ballast is therefore particularly well suited for operating four low-pressure discharge lamps that are arranged in two parallel branch circuits with in each case a series connection composed of two low-pressure discharge lamps. This variant of the ballast is, however, also suitable for operating a different number of low-pressure discharge lamps in the two parallel-connected branch circuits. For example, it is possible to arrange two series-connected low-pressure discharge lamps in the first branch circuit, and to operate only one low-pressure discharge lamp in the second branch circuit.
In accordance with a fourth variant of the invention, the ballast has electric terminals for a single low-pressure discharge lamp, or electric terminals for a plurality of series-connected low-pressure discharge lamps. Moreover, this ballast has a transformer having two windings with the same number of turns per unit length, the first windingxe2x80x94with the low-pressure discharge lamp or the low-pressure discharge lamps connectedxe2x80x94being connected directly upstream of the low-pressure discharge lamp or the series connection of the low-pressure discharge lamps, and the second winding being connected directly downstream of the low-pressure discharge lamp or the series connection of the low-pressure discharge lamps, such that the windings of the transformer are flowed through by the lamp current in opposite senses.
With the aid of this transformer and the arrangement, according to the invention, of its windings, it is ensured that the two ends of the low-pressure discharge lamp operated on the ballast shine with the same brightness even in the case of strong dimming, or that in the case of a series connection of a plurality of low-pressure discharge lamps, the low-pressure discharge lamps connected in series shine with the same brightness. The windings of the transformer cause a balancing of the current in the supply leads directly upstream and downstream of the low-pressure discharge lamp, or directly upstream and downstream of the series connection of the low-pressure discharge lamps. The losses owing to parasitic capacitances in the lamps are compensated by means of the transformer.
The ballast according to the invention is advantageously designed as an inverter with a downstream load circuit, the electric terminals for the low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit, in order to be able to operate the lamps with the aid of a high-frequency alternating current. The operation of the lamps with the aid of a high-frequency alternating current improves the luminous efficiency and enables the brightness of the lamps to be controlled in a simple way by changing the frequency of the alternating current.
In accordance with the preferred exemplary embodiments of the invention, use is made of, in particular, a half-bridge inverter whose downstream load circuit is designed as a series resonant circuit. The design of the load circuit as a series resonant circuit permits the low-pressure discharge lamps to be ignited in a simple way by means of the method of resonance increase.